RE: Re: rotor for 20v turbo engine

["Buchholz, Steven" <Steven.Buchholz@kla-tencor.com>]

> I haven't studied the firing order and valve train to see whether there is
> a
> condition where the rotor may, for example, pass a cylinder on it's intake
> stroke just before triggering spark at the next on it's power stroke.  But
> it sure seem possible that at very high RPM, with the spark advanced
> further
> and further, and a rotor which is too wide, a spark could leap to two
> cylinders simultaneously.  
> 
I would think that a single spark jumping two gaps simultaneously is quite
unlikely.  I've never opened up a distributor on a 3B, or any other 20V
turbo engine for that matter.  From the sound of things, the width at the
tip of the "bad" rotor must be approaching the spacing between the posts on
the distributor cap to cause any of the catastrophic occurrences that are
being discussed ...

You can do a fairly quick mental experiment ... the crank rotates 720
degrees for 5 cylinders, meaning that a spark plug fires every 144 degrees.
Given the way that the distributor works, the only cylinders that can be
affected by any misfire are the ones one either side of the cylinder you
intended to fire in the firing order ... the firing order itself should be
irrelevant.  What this means to me is that a misdirected spark would cause
either a wasted spark in a cylinder that is 36 degrees before BDC-power
(which I can't see causing any problem at all) or 36 degrees after
BDC-compression.  This latter case would be the one that would seemingly be
the one most likely to cause problems, but given the fact that ignition
timing is more likely to be advanced it seems to me to be the one LESS
likely to actually occur.  Perhaps the damage comes from not firing the
intended cylinder ... given a high compression engine without a spark there
may be a high likelihood of detonation if the spark does not begin the
combustion process in the intended way.  

It also seems to me that the RPMs of the engine don't enter into the
equation for rotor width ... unless the ECU provides additional advance in
this situation.  The rotor will be "in contact" with a particular tap on the
cap for a certain number of crank degrees independent of the rotation rate
of the engine.  An interesting corollary is the fact that the range of
possible advance/retard values should influence the width of the end of the
rotor.  

I am not an expert on the dynamics of internal combustion engines ... I'm
just thinking aloud on the possible failure mechanisms.  Overall I would
agree with the statement that said that for stock engines the part that is
easily available is probably more than sufficient ...

... ramble mode off ...

Steve Buchholz
San Jose, CA (USA)